1. Field of the Invention
The present invention relates to a next-generation optical information recording medium, and more particularly relates to a next-generation optical information recording medium, such as a Blu-ray Disc, using dye for a recording layer.
2. Description of the Related Art
As an information recording medium, an optical information recording medium, such as an optical disc, is about to rapidly become widespread. As the optical information recording medium as described above, for example, there may be mentioned a medium, such as a CD-R, in which a recording layer and a reflection layer are sequentially formed on a light transmissive resin substrate having a thickness of 1.2 mm and a diameter of 120 mm or 80 mm. In recent years, a higher information recording density has been required. Accordingly, a method for decreasing the wavelength of laser light and using an object lens having a higher NA (numerical aperture) has been conceived, and as a result, an optical information recording medium, such as a DVD±R, has been realized. In this DVD±R, in order to increase the permissible value of a tilt angle of the disc caused by a shorter wavelength and a higher NA, two light transmissive resin substrates having a thickness of 0.6 mm are adhered to each other with a reflection layer and a recording layer provided therebetween.
However, in recent years, in order to record high-definition image data, a higher information recording density has been required. Hence, an optical information recording medium has been proposed in which, as a Blu-ray Disc (BD-R), a reflection layer and a recording layer are formed on one surface of a resin substrate having a thickness of 1.1 mm and, on said one surface, a light transmissive cover layer having a thickness of 0.1 mm is provided.
As disclosed in Japanese Unexamined Patent Application Publication No. 2003-45079, the optical information recording medium as described above is formed to have a size equivalent to that of CD-R and/or DVD±R in which a reflection layer and a recording layer are sequentially formed on one surface of a resin substrate which has a thickness of 1.1 mm and which has a pre-groove provided in said one surface, and on the layers thus formed, a light transmissive resin-made cover layer having a thickness of 0.1 mm is formed. In addition, as disclosed in Japanese Unexamined Patent Application Publication No. 2003-36562, in order to protect a recording layer, a protective layer composed of a light transmissive inorganic material is provided between a cover layer and the recording layer in some cases.
The recording layer of the optical recording medium disclosed in the above patent documents is composed of an organic substance containing a dye, such as an azo-based dye or a cyanine-based dye, or an inorganic substance, such as Si, Cu, Sb, Te, or Ge, and pits are formed by laser irradiation for recording, so that data is recorded. However, when the same structure is used, in an optical information recording medium using an organic substance for a recording layer, there have been problems in that a jitter property and the degree of modulation are degraded. In the recording layer of an organic substance, when pits are formed by laser irradiation for recording, the amount of generated heat tends to increase. The heat thus generated may deform the substrate and the cover layer besides the recording layer. However, in order to ensure superior tracking properties, the substrate and the cover layer are generally formed from a material which is not likely to be deformed. As a result, the heat is liable to be accumulated in the recording layer, and hence it is believed that the jitter property is degraded thereby. In addition, in the recording layer of an organic substance, the change in light path length tends to decrease, and hence the degree of modulation tends to decrease. In order to increase the change in light path length, it may be conceived to increase the depth of the pre-groove. However, when the depth of the pre-groove is increased, the amount of dye for the recording layer is increased, and as a result, the amount of generated heat tends to increase. Hence, it is believed that the jitter property is further degraded.